There are three basic type of
Reflector: uses mirrors to focus the light
Refractor: uses lenses to focus the light.
Catadioptrics: uses both lenses and mirrors to
gather and focus the light.
With telescopes size matters! The telescopes light
gathering power is proportional to its objective's surface area.
As an example an 8" (20cm) telescope has 4 times the
light gathering capability then a 4" (10cm) telescope. Not 2 times as
you might first think.
That means on a clear night an 8"
scope should be able to see magnitude 14 or better stars. That's
more than 380,00 time fainter than the faintest star that can be seen
with the naked eye. The best you could do with a 4" scope would be
something around magnitude 12.5 ( The lower the number the brighter
The more light that your scope can gather the
more detail you will be able to see. This is what's called
resolution. Resolution is measured in arc seconds and is a
theoretical number but it provide you with some comparison
Back to the 4" and 8" scope comparison. A
4" scope should be able to distinguish object that are 1.2
arc seconds apart. whereas the 8" and distinguish objects that are
.6 arc seconds apart. You can see that the more light you
can gather the more detail you will be able to distinguish.
The focal length is defined ad the distance the light
travels from the objective (front lens or main mirror) to the point
where it is focused. The longer the focal length the larger the image
will be at the focal point. However, it will be fainter.
The focal ratio it the ratio of the telescopes focal
length to its aperture. Divide the focal length by the diameter of the
objective. Let say that the 8" scope had a focal length of
36", that would yield a focal ratio of f/4.5
Let me give you a few more words about the differences in telescope
The refractor telescope , as mentioned
above, employs lenses to they the light from the front of the
tube to the eyepiece. The objective usually consist of tow or
more optically configured glass that directs the light to
far end of the tube. These type of telescopes have the ability
to provide large, bright, high resolution images.
The reflector telescope has a concave
mirror at the bottom of the tube which gathers the light and focuses
it on another flat mirror suspended in the center of the front of
the tube. This mirror is set at a 45° to
send the image out the side of the tube to the eyepiece.
The catadioptric telescopes (
Schmidt-Cassegrain is a common type also known as SCT) have a
spherical mirror with a hole in the center at the back end of the
tube and a correcting lens at the other end. A mirror is mounted on
the correcting lens which directs the light back down the tube
thru the hole in the main mirror. The light is reflected at a
45° angel out the eyepiece.
Because the light travels back and forth the tube twice it
double the instrument's. focal length. Using this design, a SCT with
a 24" tube has a 48" focal length.